He has an incremental damage reduction buff on him called Lavatar. which starts at 4 increments. At about 8 increments (depending on your group set up) drag him out of the lava to let the increments drop back to 2. You cannot damage him at this point. While out of lava, adds slowly join the fight, kill the "fanatic Allu'thoa" or they will heal the named. (they get one-shotted by arrows). When he hits 2 increments again drag him back into the lava and dps. Rinse and repeat until he's dead.

As his increments increase so does his damage. Beyond 8 his pulse damage can 1 shot the group